Today, fiber producers often use a method known as solution dyeing to make "colored" nylon fibers. By the term, "colored" as used herein, it is meant a positive colorant value conferred by colored pigments and excludes the sole use of whitening agents such as TiO.sub.2 and filler materials such as talc or clay. This solution dyeing method may involve blending one or more pigments directly with the nylon fiber-forming polymer and then extruding the blend through a spinneret to make colored fibers. Alternatively, a color concentrate containing one or more pigments dispersed in a polymer matrix and also containing such additives as lubricants and delustering agents (TiO.sub.2) may first be prepared. The color concentrate is then blended with the nylon fiber-forming polymer and the blend is spun into colored fibers. For example, Anton, U.S. Pat. No. 5,108,684 involves a process where pigments are dispersed in a terpolymer of nylon 6/6,6/6,10 and pigmented pellets of the terpolymer are formed. These pellets are then remelted or "let-down" in an equal or greater amount of nylon 6, mixed thoroughly to form a uniform dispersion, resolidified, and pelletized. The resulting color concentrate is then blended with a nylon copolymer containing an aromatic sulfonate or an alkali metal salt thereof. The melt-blend is then spun to form stain-resistant, pigmented fibers.
Now, it has been found that some pigments which are introduced in neat (non-diluted) form, as described above, cause the molten polyamide to depolymerize somewhat. This causes a drop in the polymer melt viscosity which adversely affects the melt spinning process by increasing the number of filament spinning breaks and changing the cross-sectional shape of non-round filaments. In some instances, there are also problems in the spinning operation if the pigment is introduced in the form of a color concentrate. Particularly, the low molecular weight, low melt viscosity polyamide introduced with the pigment may cause the overall polymer melt viscosity to decrease sufficiently to cause spinning breaks.
In view of the foregoing, it would be desirable to have a method for introducing pigments into polyamide fibers without adversely affecting the melt spinning process. The present invention provides such a method along with the resultant fibers.